In the geometry optimization and molecular dynamics simulations, the
restart files generated at the previous steps are automatically utilized
at the next step to accelerate the convergence using
an extrapolation scheme [42,43]. In the extrapolation scheme,
the number of previous MD or geometry optimization steps can be controlled by
a keyword:

scf.ExtCharge.History 2 # default=2

From a series of benchmark calculations, 'scf.ExtCharge.History' of 2 works well
and a larger number tends to be numerically unstable. So, users are recommended
to use the default setting of 2.